The invention relates to an installation for producing solar thermal energy, having an absorber layer for conversion of sunlight into thermal energy and a transparent heat-carrying liquid for transporting the heat produced in said absorber layer for further use, said absorber layer being situated within said heat-carrying liquid.
Installations for producing solar thermal energy are known in various conformations. Examples include solar tower power plants or parabolic trough power plants, which make solar energy usable by thermal conversion on a commercial scale. However, such power plants have very large surface area requirements. In addition, the use of same is limited to latitudes where high direct solar radiation is assured, because they can make little use of the diffuse portion of the sunlight.
On a smaller scale, facilities for producing solar thermal energy are known in the form of solar collectors for residential water heating, said collectors preferably being installed on the roof. However, such a decentralized use of solar energy serves more to reduce residential energy costs than to replace fossil energy fuel as the primary energy source.
A floating solar power plant for the commercial-scale production of electricity is known from Patent DE 25 43 687 C2. The surfaces of said solar power plant are designed as absorbers and track the sun""s path. Said absorbers are hereby mounted on a floating collector platform, designed as a circular plate of very large diameter, made from a flexible material designed to absorb tensile stress. Said floating collector platform is provided with devices that can turn the platform on the water.
Patent DE 28 19 946 A1 describes a thermal storage pond and an energy generating installation using same. Said thermal storage pond contains a thermal storage liquid that floats upon an insulating layer, said layer being thick enough to thermally insulate said thermal storage liquid. Said energy generating installation encompasses said thermal storage pond. In addition, a thermal engine is provided to extract heat from said thermal storage liquid.
A solar collector having an absorber situated within a transparent heat-carrying liquid is known from Utility Model DE 296 03 275 U1. This specialized arrangement of said absorber is intended to minimize heat loss from said absorber to said heat-carrying liquid during heat transfer, and in particular to avoid radiation loss from said absorber. However, the material costs for this known solar collector are relatively high, so that use is limited to residential water heating.
Proceeding from the prior art, the object of this invention is to provide an installation for producing solar thermal energy, using the aforementioned principle of the absorber arrangement, to make solar energy usable by thermal conversion on a commercial scale, without additional fuel firing. This objective is achieved by an installation having the features of accompanying claim 1.
Advantageous developments and further developments of the invention arise from claims 2-19.
The main element of the installation according to the invention is at least one thermal storage water heater wherein hot water at a temperature typically around 100xc2x0 C. is produced from solar radiation and simultaneously stored, using an absorber layer on the inside thereof. Said thermal storage water heater is situated to float beneath a water surface. The installation according to the invention is thus set up in a body of water, preferably in an ocean, so that no land area is consumed. Solar radiation is directly converted to heat in the energy storage unit, thus eliminating energy loss during heat transport. Use of integrated energy storage removes the need for fossil fuels during the time when solar radiation is absent.
The thermal storage water heater is provided with thermal insulation to reduce heat loss to the surroundings. Said thermal insulation is transparent on the upper side of said thermal storage water heater to allow solar radiation to penetrate to the absorber layer.
Hot water can be removed from the thermal storage water heater as a heat-carrying liquid, with which electricity or drinking water can be produced without additional fuel firing at middle to base load conditions. Heat decoupling for supplying district heating, for example, is also possible.
One advantage of the invention is that the installation may be operated for producing solar thermal energy outside daylight hours, because sufficient energy can be stored at night and in times of low solar radiation by appropriate sizing of the thermal storage water heater. Depending on the size of the installation and the climate zone of the installation site, economic operation at middle to base load conditions is possible.
In contrast to known parabolic trough power plants, the installation according to the invention can use not only parallel solar radiation but also the diffuse portion thereof. Thus, said installation may be used even in tropical and moderate climate zones, thereby greatly increasing the areas of use.
The concept according to the invention, to transfer the heat generated in the absorber directly to the contents of the thermal storage water heater and to remove the heat-carrying liquid from said thermal storage water heater for further use, is particularly usable on a commercial scale due to the floating design of said thermal storage water heater. Said thermal storage water heater may be simply constructed in sizes that assure efficient energy storage.
A variable-volume design of the thermal storage water heater offers further advantages: depending on the intensity of solar radiation, the storage capacity can be modified. When the incident energy radiation is high, said thermal storage water heater can be enlarged by addition of colder water, thus eventually heating the colder water to the storage temperature with no loss of excess energy. When the installation is operated in the absence of solar radiation (at night), hot water is removed from said storage unit, thus decreasing the storage volume. Thus, colder water must not flow back into said thermal storage water heater, so that the storage temperature can remain unchanged at its high level, even during nighttime operation of said installation.
It is advantageous to provide the thermal storage water heater with membrane-like, movable walls. This not only conserves materials, but also avoids static difficulties and stability problems with the storage unit wall. Finally, in this manner water movements may be intercepted to a certain degree without risking damage to the storage unit walls.
Since most processes using the heat removed from the thermal storage water heater do not cool the hot water to ambient temperature, it is advantageous to provide a warm water recirculation heater for the returning heat-carrying liquid, said warm water recirculation heater being installed adjoining said thermal storage water heater. If the interface between the two said heaters is sufficiently large, heat loss from said thermal storage water heater to the surroundings is significantly reduced. Heat is transferred from said thermal storage water heater to said warm water recirculation heater more slowly due to the smaller temperature gradient, so that the heat flowing into said warm water recirculation heater is still not lost for the utilization process.
Preferably, the thermal storage water heater and the warm water recirculation heater can be designed as a module with common outer walls, whereby a movable partition separates the storage contents from one another. The movable design of said partition, which can be composed of thermal insulation, allows the volume of said thermal storage water heater and of said warm water recirculation heater to be modified, thus offering advantages depending on the solar radiation and the operating situation.
Such a partition designed as thermal insulation is preferably produced in a module as a single unit with the absorber layer, wherein the density of the entire structure is adjusted to be slightly greater than the density of the contents of the thermal storage water heater at the desired end temperature. Floating of said partition and said absorber layer is thus reliably prevented, thereby allowing said partition to be moved even by self-regulating means without outside intervention.
Reduction of heat loss from the thermal storage water heater is especially pronounced for thermal storage water heaters of a flat design, when said thermal storage water heater is embedded, at least at its upper and lower sides, into the warm water recirculation heater. Consequently, a portion of the incident solar energy absorbed approximately in the top-situated warm water recirculation heater is not lost for the utilization process, since said portion of incident solar energy additionally preheats the water before said water reaches said thermal storage water heater.
It can be advantageous if the portion of the warm water recirculation heater situated above the thermal storage water heater has the shape of a convex lens, thus allowing the parallel portion of the solar radiation to be concentrated in a specific region of said thermal storage water heater, wherein it is practical to arrange at that location a high-efficiency absorber.
Instead of or in addition to the design of the warm water recirculation heater as a convex lens, film panels may also be provided in the thermal storage water heater and preferably arranged in a parabolic shape at a particular region of said thermal storage water heater, thus concentrating the incident sunlight on this region.
In particular when the majority of the thermal storage water heater is embedded in the warm water recirculation heater, for the aforementioned reasons it is advisable that the former have membrane-like, movable walls.
The heat energy generated in the installation according to the invention and removed from the thermal storage water heater can be used advantageously for further purposes by (at least partially) evaporating in a steam generator the heat-carrying liquid removed from said thermal storage water heater. A condenser for condensing the steam can be installed following such a steam generator, wherein between said steam generator and said condenser is installed in a known manner a steam turbine having a generator for electric power generation. Drinking water may also be produced from the distillate of said condenser by means of a drinking water processing plant.
When the installation according to the invention is used in coastal regions, a seawater desalinization plant can be operated directly from the heat-carrying liquid removed from the thermal storage water heater. Advantageously, said seawater desalinization plant is installed in addition to electric power generation, and installed following the steam generator.
For mechanical protection of the installation according to the invention, in particular for use in the ocean, it is advantageous to cover said installation, or at least the thermal storage water heater, with a layer of surface water to provide protection from all types of wind effects. In particular, wave formation at the ocean surface during storms is then limited to the layer of surface water above said installation according to the invention, thus eliminating the danger of damage to said installation when said thermal storage water heater and optionally the warm water recirculation heater are equipped with membrane-like, movable walls. The transparent thermal insulation situated on the upper side of said thermal storage water heater is protected just as effectively.
For commercial-scale use of the invention, it is advisable to adjacently arrange a plurality of thermal storage water heaters of modular construction, and to connect same with a collecting line for removal of the heat-carrying liquid. In this regard it is particularly advantageous if said collecting line is designed as a steam generator so that process steam can be centrally removed from the installation of modular design.
For larger installations, a multipressure process can offer economic advantages. The seawater is hereby led to firther evaporation stages after leaving the first evaporation stage, thereby experiencing greater cooling than with the single-pressure process. In this manner, the enthalpy of the steam from said first stage, and thus the process efficiency can be increased.